linux-kernel - Re: [PATCH v2] sched: fix nohz.next

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Message-ID: <CAKfTPtAb8YEQx-dHCatnqMWM_GckAJsCTx9EcAaHnyjddUZThw@mail.gmail.com>
Date:	Fri, 28 Aug 2015 13:14:12 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v2] sched: fix nohz.next_balance update

On 27 August 2015 at 18:50, Jason Low <jason.low2@...com> wrote:
> On Thu, 2015-08-27 at 13:21 +0200, Vincent Guittot wrote:
>> Hi,
>>
>> On 3 August 2015 at 11:55, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>> > Since commit d4573c3e1c99 ("sched: Improve load balancing in the presence
>> > of idle CPUs"), the ILB CPU starts with the idle load balancing of other
>> > idle CPUs and finishes with itself in order to speed up the spread of tasks
>> > in all idle CPUs.
>> >
>> > The this_rq->next_balance is still used in nohz_idle_balance as an
>> > intermediate step to gather the shortest next balance before updating
>> > nohz.next_balance. But the former has not been updated yet and is likely to
>> > be set with the current jiffies. As a result, the nohz.next_balance will be
>> > set with current jiffies instead of the real next balance date. This
>> > generates spurious kicks of nohz ilde balance.
>> >
>> > nohz_idle_balance must set the nohz.next_balance without taking into
>> > account this_rq->next_balance which is not updated yet. Then, this_rq will
>> > update nohz.next_update with its next_balance once updated and if necessary.
>> >
>> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>
> Acked-by: Jason Low <jason.low2@...com>

Thanks

>
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